Over the past, thyristor inverters or transistor inverters have been employed as a high frequency energy source of induction heating apparatus in which a magnetic material is heated by eddy currents generated therein as a result of a high frequency magnetic field produced by a heating coil. Output power control methods which have been proposed are classified broadly into three categories. A first method involves carying the d.c. power voltage of the inverter, a second method involves periodically inhibiting the inverter which is called "duty cycle control", and the third method is to control the oscillating frequency of the inverter (ON-OFF duty control). However, the first method has resulted in a high apparatus cost, the second method has presented a lamp flicker problem and the problem of lengthened warmup periods, and the third method has also presented problems in that it required the use of a heavy duty power switching transistor to bear the burden of transient surge currents and high potentials. In the case of thyristor inverters, in particular, output power is variable as a function of the oscillating high frequency. Theoretically, the power loss of the switching power transistor can be decreased by lowering the oscillating frequency. However, this is achieved only at the expense of a noise generated when the frequency becomes lower than the inaudible limit. Therefore, the inaudible frequency limit sets the lower limit of power control range. Whereas, the allowable values of the surge current and high potential for the thyristor set the upper limit on the variable frequency range and hence the upper limit of the power control range. Thus, a wide range of power control was impossible with the prior art thyristor inverters. On the other hand, the problem associated with transistor inverters is concerned with difficulty in providing a wide range power control without imposing heavy burden on the switching transistor.